Traces have typically been used to obtain information about a reservoir and/or about what is taking place therein. In particular, tracers may be used to label fluids that are injected into a specified reservoir in order to track fluid movement and fluid velocities, as well as monitor chemical changes of the injected fluid. U.S. Pat. No. 5,246,860, for example, teaches tracer chemicals for use in monitoring subterranean fluids, e.g. geothermal brines) and is herein incorporated by reference in its entirety.
U.S. Pat. No. 4,555,488 provides another method for utilizing tracer chemicals to determine flow patterns in subterranean petroleum and mineral containing formations using organonitrogen tracers, and is herein incorporated by reference in its entirety. In recovery of petroleum or minerals from subterranean formations, especially by chemical flooding, it is desirable to know the flow patterns of the formation prior to injection of chemicals. Tracers are used in such reservoir engineering. The tracer is generally water soluble and inert to the solids and liquids present in the formation (e.g. it does not get absorbed onto the rocks; it does not partition into any oil phase which may be present; and it does not interact with the organics and minerals present in the formations).
Another common use for tracers is with regard to hydraulic fracturing. Hydraulic fracturing is a well-established technique for stimulating production from a hydrocarbon reservoir. Typically a thickened, viscous fracturing fluid is pumped into the reservoir formation through a wellbore and fractures the formation. Thickened fluid is then also used to carry a particulate proppant into the fracture. The fracturing fluid is subsequently pumped out and hydrocarbon production is resumed. As the fracturing fluid encounters the porous reservoir formation a filtercake of solids from the fracturing fluid builds up on the surface of the rock constituting the formation. After fracturing has taken place a breaker (which is usually an oxidizing agent, an acid or an enzyme) may be introduced to break down this filter cake and/or to reduce the viscosity of the fluid in the fracture and allow it to be pumped out more effectively. Tracers may be used in connection with this hydraulic fracturing procedure, mainly to provide information on the location and orientation of the fracture, as described in U.S. Pat. No. 3,987,850. U.S. Pat. No. 3,796,883 describes a further use of radioactive tracers to monitor the functioning of a well gravel pack.
Additionally, tracers may be introduced into the reservoir using various known methods. For instance, tracers may be associated with a carrier material (e.g. particles) from which the tracer is released after the carrier material is placed in a subterranean reservoir and/or exposed to the contents therein. U.S. Pat. No. 6,723,683 describes using starch particles as a carrier for a variety of oilfield chemicals including tracers. U.S. Pat. Nos. 7,032,662 and 7,347,260 also describe the association of a tracer substance with a carrier. U.S. Pat. Pub. No. 2010/0307745 further describes the use of encapsulated tracers and is herein incorporated by reference in its entirety.
Moreover, U.S. Pat. No. 5,892,147 discloses a procedure where, during the manufacture of a well, a plurality of different tracer substances are placed at respective locations along the length of a well penetrating a reservoir prior to completion of the well. When the manufacture of the well is completed and production commences, the individual tracers may be monitored in order to calculate the proportions of oil or gas being flowing into the well from the reservoir. U.S. Pat. No. 6,645,769 also provides that multiple tracers (associated with carrier particles) should be located at respective zones of a reservoir and/or injection well during completion of the injection well. Specifically, this patent describes dividing regions around wells in the reservoir into a number of zones/sections and immobilizing tracers on a filter, a casing or other such construction surrounding the injection well in different zones/sections.
Typically, tracers comprise distinctive chemicals, which may be detected in high dilution, such as fluorocarbons, dyes or fluorescers. Genetically coded material has also been proposed as a possible tracer (e.g. WO2007/132137 provides a method for detection of biological tags). However, modern tracers generally comprise radioactive isotopes (e.g. Society of Petroleum Engineers paper SPE 109,969 discloses the use of materials which can be activated to become short lived radioactive isotopes). Such radioactive isotopes may include potassium iodide, ammonium thiocyanate, dichromate, etc. Unfortunately radioactive isotopes are expensive and require special handling by licensed personnel because of the danger posed to personnel and the environment. Another drawback to using radioactive isotopes is the alteration by the radioactive materials of the natural isotope ratio indigenous to the reservoir, thereby interfering with scientific analysis of the reservoir fluid characteristics. The half-life of radioactive tracers also tends to be either too long or too short for practical use. In addition, certain radioactive isotopes, such as potassium iodide, may be limited to wet analyses type detection methods.
Accordingly, despite the importance of tracers in tracking the movement and/or characteristics of fluids in reservoirs, very few suitable tracers are presently available. Furthermore, of those that are available, little is known about their stabilities or behavior in specific environmental conditions.